CN111344636B

CN111344636B - Marking, overlay target and alignment and overlay method

Info

Publication number: CN111344636B
Application number: CN201880069291.6A
Authority: CN
Inventors: B·夏卡; S·拉尔巴哈多尔辛; 王甲
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2017-10-24
Filing date: 2018-08-23
Publication date: 2022-02-15
Anticipated expiration: 2038-08-23
Also published as: JP2021501351A; US20200319563A1; KR20200053604A; TW201923491A; KR102390742B1; TWI692680B; US11086232B2; WO2019081091A1; CN111344636A; JP7279032B2

Abstract

A resonant amplitude grating marker has a periodic structure configured to scatter radiation (502) of wavelength λ incident (500) on a surface plane (506) of an alignment marker. Scattering is achieved primarily by exciting resonant modes (508) in the periodic structure parallel to the surface plane. Effective refractive index (n) of portions of periodic structure_s,n_d) And length (L)₁,L₂) Configured to provide an optical path length (n) of the unit cell in the periodic direction_sL₁+n_dL₂) The optical path length is equal to an integer multiple (m λ) of the wavelengths present in the radiation spectrum. Effective refractive index (n) of these portions_s,n_d) And length (L)₁,L₂) Is further configured to provide an optical path length (n) of the second portion in the periodic direction_dL₂) The optical path length is equal to half the wavelength (lambda/2) present in the radiation spectrum.

Description

Marking, overlay target and alignment and overlay method

Cross Reference to Related Applications

The present application claims priority from EP application 17197914.9 filed on 24.10.2017 and EP application 18170352.1 filed on 2.5.2018, the entire contents of which are incorporated herein by reference.

Technical Field

The present disclosure relates to a marking, overlay target, and related methods of aligning and determining overlay errors, such as may be used in the manufacture of devices by lithographic techniques.

Background

A lithographic apparatus is a machine that applies a desired pattern onto a substrate. Lithographic apparatus can be used, for example, to manufacture Integrated Circuits (ICs). A lithographic apparatus may, for example, project a pattern (also commonly referred to as a "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

To project a pattern on a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm and 13.5 nm. Lithographic apparatus using Extreme Ultraviolet (EUV) radiation having a wavelength in the range 4nm to 20nm (e.g. 6.7nm or 13.5nm) may be used to form smaller features on a substrate than lithographic apparatus using radiation having a wavelength of, for example, 193 nm.

Low k1 lithography can be used to process features having dimensions smaller than the classical resolution limit of the lithographic apparatus. In this process, the resolution formula may be expressed as CD — k1 × λ/NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (typically the minimum feature size for printing, but in this case half pitch), and k1 is the empirical resolution factor. In general, the smaller k1, the more difficult it is to reproduce a pattern on a substrate that is similar in shape and size to that planned by a circuit designer for achieving a particular electrical function and performance. To overcome these difficulties, complex trimming steps may be applied to the lithographic projection apparatus and/or the design layout. These include, for example, but are not limited to, optimizing NA, customizing illumination schemes, using phase-shifting patterning devices, various optimizations of the design layout such as optical proximity correction (OPC, also sometimes referred to as "optical and process correction") in the design layout, or other methods commonly defined as "resolution enhancement techniques" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus may be used to improve the reproduction of the pattern at low k 1.

Accurate placement of patterns on a substrate is a major challenge in reducing the size of circuit components and other products that may be produced by photolithography. In particular, the challenge of accurately measuring features on an already-laid substrate is the critical step of being able to produce a working device with high yield in sufficiently accurate alignment of successive layers of superimposed features. In general, in today's submicron semiconductor devices, the so-called overlap should be achieved within tens of nanometers, and in the most critical layers should be down to a few nanometers.

In lithographic processes, measurements of the formed structures are often required, for example, for process control and verification. Various tools are known for making such measurements, including scanning electron microscopes, which are commonly used to measure Critical Dimension (CD), and specialized tools for measuring the overlay, the alignment accuracy of two layers in a device. In recent years, various forms of scatterometers have been developed for use in the field of lithography. These devices direct a beam of electromagnetic radiation onto a target and measure one or more characteristics of the scattered electromagnetic radiation (e.g., intensity at a single reflection angle as a function of wavelength, intensity at one or more wavelengths as a function of reflection angle, or polarization as a function of reflection angle) to obtain a diffraction "spectrum" from which a characteristic of interest of the target can be determined.

Conventional alignment marks consist of binary phase gratings that diffract incident radiation. They rely on constructive interference of radiation diffracted by the grating top and grating bottom at the optimal grating depth. This light is then captured by the alignment sensor and used to define the location of the marks on the wafer. In an ideal scenario where the alignment marks are perfectly symmetrical, the best overlay is obtained assuming no wafer deformation, zero Alignment Position Deviation (APD). However, as a result of processing such as etching, Chemical Mechanical Polishing (CMP), annealing, deposition, oxidation, and the like, the actual alignment marks are deformed in various ways, often resulting in an asymmetry that is not known in advance. Typical asymmetries observed include Floor Tilt (FT), roof tilt (TT), and sidewall angle (SWA). Further, the depth of the alignment mark may also vary around the nominal value due to fluctuations in the processing.

When radiation from the alignment sensor interacts with the alignment marks and diffracts, this diffracted radiation also contains information about the mark geometry. Thus, for an asymmetric (distorted) alignment mark, the position detected by the sensor is different from the actual position on the wafer: APD is not zero. This results in overlay errors that depend largely on the type and magnitude of asymmetry induced in the mark and also on the mark depth.

If the details of the effect of the process are known in advance or after investigation, alignment marks that are less sensitive to deformation during a particular process may be used. However, these marks are highly specific to certain processes and semiconductor manufacturers. The standard method comprises the following steps: by performing the alignment with different colors, one color or a combination of colors that minimizes APD can be identified for a particular mark deformation, since the information about the mark profile contained in the diffracted radiation depends to a large extent on the color. However, such fluctuations in the deformation of the mark exist across the wafer and vary from wafer to wafer. As a result, even in scenes using multiple colors, the overlay performance is not optimal. Still further, some alignment systems are limited to only two colors.

Overlay targets used to measure overlay error also suffer from the same distortion problem. This may result in inaccurate overlay errors of the measurements.

Disclosure of Invention

It is desirable to have a mark and an overlay target that are less sensitive to mark asymmetry caused by process induced distortion.

According to a first aspect of the present invention there is provided a marker formed on a planar substrate, the marker comprising a periodic structure configured to scatter radiation incident on a surface plane of the alignment marker, the surface plane being parallel to the plane of the substrate, the scattering being achieved primarily by excitation of resonant modes in the periodic structure that are parallel to the surface plane.

According to a second aspect of the present invention there is provided a substrate comprising the marking of the first aspect.

According to a third aspect of the present invention there is provided an overlay target comprising a lower mark according to the first aspect, the lower mark overlapping an upper mark, the upper mark having the same pitch as the lower mark and comprising a periodic structure configured to scatter radiation without exciting in the periodic structure a resonant mode parallel to its surface plane on which the radiation is incident.

According to a fourth aspect of the present invention there is provided a substrate comprising the overlay target of the third aspect.

According to a fifth aspect of the present invention, there is provided an alignment method comprising the steps of:

-providing an alignment mark formed on a planar substrate, the alignment mark comprising a periodic structure configured to scatter radiation incident on a surface plane of the alignment mark, the surface plane being parallel to the plane of the substrate, the scattering being achieved primarily by exciting resonant modes in the periodic structure that are parallel to the surface plane;

-illuminating the alignment mark with radiation;

-detecting radiation scattered by the alignment mark resulting from the illumination; and

-determining the position of the alignment mark using the detected radiation.

According to a sixth aspect of the present invention, there is provided a method of determining an overlay error, comprising the steps of:

-providing an overlay target formed on a planar substrate, the overlay target comprising a lower mark which overlaps an upper mark having a pitch which is the same as the pitch of the lower mark, wherein

The lower mark comprises a periodic structure configured to scatter radiation incident on a surface plane of the lower mark, the surface plane being parallel to the plane of the substrate, the scattering being mainly achieved by exciting resonant modes in the periodic structure parallel to its surface plane; and

the upper marker comprises a periodic structure configured to scatter the radiation without exciting resonant modes in its periodic structure with its surface plane on which the radiation is incident;

-illuminating the overlapping targets with radiation;

-detecting radiation scattered by overlapping targets caused by the illumination; and

-determining an overlay error between the upper mark and the lower mark using the detected radiation.

Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which

FIG. 1 depicts a schematic overview of a lithographic apparatus;

figure 2 depicts the diffraction of a conventional phase grating;

figure 3 depicts the diffraction of an amplitude grating;

figure 4 depicts the grating in a resonant regime;

fig. 5 depicts a resonance amplitude mark according to an embodiment of the invention;

fig. 6 depicts an electric field simulation of a phase grating similar to the one shown in fig. 2;

FIG. 7 depicts an electric field simulation of a resonant amplitude grating similar to the resonant amplitude grating shown in FIG. 5;

FIG. 8 depicts a graph of the diffraction efficiency of the +1 diffraction order as a function of grating mark depth and optical path length;

figure 9 depicts the Alignment Position Deviation (APD) as a function of the alignment mark depth in the presence of the floor tilt asymmetry of a conventional phase grating and two resonant amplitude gratings, respectively;

FIG. 10 depicts the Wafer Quality (WQ) as a function of the alignment mark depth in the presence of floor tilt asymmetry for a conventional phase grating and two resonant amplitude gratings, respectively;

figure 11 depicts the Alignment Position Deviation (APD) as a function of the alignment mark depth in the presence of side wall angle asymmetries of a conventional phase grating and two resonant amplitude gratings, respectively;

FIG. 12 depicts the Wafer Quality (WQ) as a function of the alignment mark depth in the presence of sidewall angle asymmetries for a conventional phase grating and two resonant amplitude gratings, respectively;

figure 13 depicts the Alignment Position Deviation (APD) as a function of the alignment mark depth in the presence of top tilt asymmetry of a conventional phase grating and two resonant amplitude gratings, respectively;

figure 14 depicts the Wafer Quality (WQ) as a function of the alignment mark depth in the presence of top tilt asymmetry of a conventional phase grating and two resonant amplitude gratings, respectively;

FIG. 15 depicts a resonant amplitude grating with a unit cell of a sub-divided part according to an embodiment of the invention;

fig. 16 depicts a stack of marks with resonant amplitude marks on top according to an embodiment of the invention;

FIG. 17 depicts an overlay target with bottom resonance amplitude marks according to an embodiment of the present invention;

FIG. 18 is a flow chart of an alignment method according to an embodiment of the invention;

FIG. 19 is a flow chart of an overlay error measurement method according to an embodiment of the invention;

FIG. 20 depicts a super-wavelength sub-divided resonance amplitude marker according to an embodiment of the invention;

FIG. 21 depicts a sub-wavelength sub-division phase signature compared to the super-wavelength sub-division resonance amplitude signature of FIG. 20;

figure 22 depicts Alignment Position Deviations (APDs) as a function of alignment mark depth for the super-wavelength sub-division resonance amplitude mark of figure 20 and the sub-wavelength sub-division phase mark of figure 21; and

fig. 23 depicts Wafer Quality (WQ) as a function of alignment mark depth for the super-wavelength sub-division resonance amplitude mark of fig. 20 and the sub-wavelength sub-division phase mark of fig. 21.

Detailed Description

FIG. 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as an illuminator) IL configured to condition an electromagnetic radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation); a support structure (e.g. a mask table) T configured to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

In operation, the illuminator IL receives a radiation beam from a radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in cross-section at the plane of the patterning device MA.

The term "projection system" PS as used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic (anamorphic), magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered as synonymous with the more general term "projection system" PS.

The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g., water) so as to fill a space between the projection system and the substrate, which is also referred to as immersion lithography. More information on immersion techniques is given in U.S. Pat. No. 6,952,253 and PCT publication No. WO99-49504, which are incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two (dual stage) or more substrate tables WT and, for example, two or more support structures T (not shown). In such "multiple stage" machines the additional tables/structures may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used to expose the design layout of the patterning device MA onto the substrate W.

In operation, the radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table T) and is patterned by the patterning device MA. After traversing the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder, 2D encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and (possibly) another position sensor (which is not explicitly depicted in fig. 1) can be used to accurately position the mask MA with respect to the path of the radiation beam B. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are referred to as scribe-lane alignment marks).

Embodiments of the present invention provide novel marks that are made as binary gratings. These use surface mode coupling and leakage. A general framework of mark design insensitive to asymmetry is provided. A general framework of mark design that is insensitive to depth variations is also provided. The novel marker design requires only a single wavelength to mitigate the effects of process induced marker asymmetry. Further, the alignment signal strength (wafer quality WQ) can be adjusted simply by adjusting the pitch of the marks. These marks simplify the fabrication of the reference wafer, providing a "golden" reference wafer for "wafer-to-wafer error correction" because they are insensitive to process-induced mark asymmetries.

Embodiments provide a new binary marker design that is insensitive to most types of asymmetries (FT, SWA). It is insensitive to depth variations and therefore to process fluctuations of any asymmetry (FT, SWA, TT).

Before considering the novel labels, a conventional phase grating (fig. 2) will be described, as will the principle of an amplitude grating (fig. 3) and the generation of resonances (fig. 4).

Figure 2 depicts diffraction of a conventional phase grating. Radiation 200 having a wavelength λ illuminates a periodic structure 210 formed on a planar substrate 212, the periodic structure 210 being a grating shown in cross-section in this example. The gaps between the ridges 210 form trenches of depth d that extend down to the substrate 212. The (d) is the mark depth. The interference between the

scattered radiation

204, 206 reflected at the top and bottom of the grating 210, respectively, is constructive for producing the optimal thickness d of the scattered radiation 202. Therefore, diffraction occurs via modulation of the phase of the reflected wave. The grating introduces a periodic modulation of the wavefront.

Figure 3 depicts diffraction by an amplitude grating. Radiation 300 of wavelength lambda illuminates grating 310. The radiation 302 diffracted by the grating with periodic holes in the reflective film depends only on the period Λ. The reflective film is equivalent to a collection of point sources 304. In contrast to a phase grating, diffraction occurs by modulation of the amplitude, not the phase, of the reflected wave. Like the phase grating, the amplitude grating introduces a periodic modulation of the wavefront.

Figure 4 depicts the grating in a resonant mechanism. Radiation 400 having a wavelength λ illuminates grating 510. The incident radiation 400 is resonantly excited into a counter-propagating wave 408 in the plane of the grating. The grating itself introduces the required momentum.

Fig. 5 depicts a resonant amplitude mark according to an embodiment of the invention. The marks are formed on a planar substrate 512. The mark has a periodic structure configured to scatter radiation 502 of wavelength λ incident 500 on a surface plane 506 of the alignment mark. The surface plane 506 is parallel to the plane of the substrate. Scattering is achieved primarily by exciting resonant modes 508 in the periodic structure parallel to the surface plane.

The periodic structure has repeating unit cells divided into adjacent first and

second portions

510, 504 along a periodic direction (left to right in the cross-section of fig. 5).

The first portion 510 has a first effective index (n)_s) And a first length (L) along the periodic direction₁). The second portion 504 has a second effective index of refraction (n) that is lower than the first effective index of refraction in its optical path_d) And a second length (L) along the periodic direction₂)。

Effective refractive index (n) of these portions_s，n_d) And length (L)₁，L₂) Configured to provide an optical path length (n) of the unit cell in the periodic direction_sL₁+n_dL₂) The optical path length is equal to an integer multiple (m λ) of the wavelengths present in the radiation spectrum.

The wavelength of the incident radiation may be predetermined such that it matches the resonant design rules. Alternatively, broadband radiation may be incident on the indicia, and then the alignment sensor frequency filter may be tuned to select the resonant mode wavelength.

Effective refractive index (n) of these portions_s，n_d) And length (L)₁，L₂) Is further configured to provide an optical path length (n) of the second portion in the periodic direction_dL₂) The optical path length is equal to half an integer multiple of the wavelengths present in the radiation spectrum (k λ/2). These are conditions that match the wavelength of the radiation to the grating material boundary conditions to support resonance.

In this example, the degree of optical path length of the second portion in the periodic direction (n)_dL₂) Equal to half the wavelength (lambda/2) present in the radiation spectrum, so that only one antinode of the resonant mode is present in the second part 504, i.e. k is 1. When k is>1, there are an odd number of antinodes, but an even number cancel out, leaving only one antinode that contributes to the scattering but is less efficient.

Marks formed on a substrate such as a wafer WLike planar substrates, e.g. P in FIG. 1₁And P₂As depicted.

The radiation diffracted by the marks does not contain information about the mark profile but only about the position of the marks on the wafer. This mark may be referred to as a Resonance Amplitude Mark (RAM). The term was chosen to highlight the different working principles of such a RAM with respect to conventional alignment marks based on phase gratings (as described with reference to fig. 2). "Mark" and "grating" may be used interchangeably. The grating may be a one-dimensional (1D) grating, as described with reference to the example of fig. 5, but the invention is not limited to 1D gratings. The invention can be applied to 2D gratings where the length and effective index of refraction in both periodic directions are configured to support resonance.

In this marker design, radiation from the alignment sensor excites two counter-propagating waves in the grating plane. These two waveforms form a so-called "standing wave", i.e. a resonant mode in the plane of the grating. As described with reference to fig. 4, the two counter-propagating waves do not propagate through the mark depth, but remain on the mark surface and are therefore not affected by the mark depth.

This resonant mode, like any other type of marker, effectively leaks light into the grating steps and can be captured in exactly the same way as a conventional marker, thus not requiring a new sensor design. In fact, the sensor will not be able to distinguish whether the light is coming from the RAM or from the conventional marker, with the advantage that the light from the RAM does not contain information about the marker profile, but only about the marker position, since the resonance mode is located in the grating plane. In fact, the mark appears as an amplitude mark, the radiation characteristics of which are independent of the mark depth, in the sense that the light diffracted in the grating orders comes in a periodic manner from a point light source located on the grating surface. This is similar to a scene where a periodic slit is opened in a reflective opaque film, as described with reference to fig. 3.

By proper design, this makes the mark much less sensitive to the layer stack present below it, and it can also be used as an overlay target (see fig. 17) or for marking the stack (see fig. 16).

For RAM configurations, it is desirable to have an effective coupling to the resonant mode and an effective leakage of that mode in the sensor plane.

These can be provided by using the following design rules:

the optical path length of the unit cell of the grating is equal to an integer multiple of the wavelength:

n_sL₁+n_dL₂＝mλ (1)

and the optical path length of the low refractive index material (space) is equal to half the wavelength:

n_dL₂＝kλ/2， (2)

wherein L is₁+L₂Λ (pitch) and k are integers, preferably k is 1. According to these two simple design rules, different pitches can be used for a specific color, depending on the sensor specifications (numerical aperture, NA). Thus, for a fixed wavelength λ, an increase in the mark pitch Λ may result in a larger duty cycle.

Fig. 6 depicts an electric field simulation of a phase grating similar to that shown in fig. 2. The lighter shading indicates higher electric field strength (see bars on the right side of the figure).

The highest intensity within the periodic structure of the grating is emitted in-situ in the trench at a Z-position (vertical axis) of about 0.2 μm. The simulation used gratings with a mark depth of 0.3 μm, a period of 2000nm and a far infrared wavelength of 850 nm.

Figure 7 depicts a simulation of the electric field of a resonant amplitude grating similar to that shown in figure 3. Resonant modes excited in the grating plane, which are standing waves, leak scattered radiation into the grating steps, independent of the depth of the marks. This is because the highest strength of the field associated with the periodic structure of the grating is found outside the trench at a Z position (vertical axis) of about 0.4 μm. This field is insensitive to the depth of the marks and to trench asymmetry. The simulation used gratings with a mark depth of 0.3 μm, a period of 1942nm, and a far infrared wavelength of 850 nm.

FIG. 8 depicts a graph of diffraction efficiency for the +1 diffraction order as a function of normalized grating mark depth d/λ (vertical axis) and optical path length (horizontal axis).

When the conditions (1) and (2) are satisfied, the diffraction efficiency (wafer quality, WQ) is independent of the mark depth of the mark. When m is equal to n_sL₁+L₂This is illustrated by the vertical band at 4. In this example, n_d＝1。

Fig. 9 depicts an Alignment Position Deviation (APD) as a function of alignment mark depth in the presence of backplane tilt (FT) asymmetry for a conventional phase grating and two resonant amplitude gratings, respectively. The inset in fig. 9 illustrates the shape of the floor tilt asymmetry. In this case, the floor tilt is a depth difference of 1nm from one side of the trench to the other. The conventional phase grating has a pitch of 3.2 μm and its pattern has square marks. The resonant amplitude grating has pitches of 1.94 μm and 3.11 μm, respectively, with the figures having circular and triangular marks, respectively. The wavelength was 850 nm.

Fig. 10 depicts Wafer Quality (WQ) as a function of alignment mark depth in the presence of floor tilt asymmetry for a conventional phase grating and two resonant amplitude gratings, respectively.

As can be seen from fig. 9 and 10, the effect of FT on APD is of a smaller order of magnitude for the resonant amplitude grating (the graph with the circular and triangular marks) than for the conventional phase grating (the graph with the square marks). For a resonant amplitude grating (graph with circular and triangular marks), WQ is still high enough to be easily detectable (>20%). Note that if desired, WQ can be further increased by using a smaller m in equation 1. This may be achieved by reducing the mark pitch a and/or the ridge (L) to the mark₁) Depth sub-wavelength sub-segmentation is performed, as described with reference to fig. 15. Furthermore, using a smaller pitch may allow for smaller markers to be used, thereby facilitating savings in real estate for the scribe lane.

Still further, for both APD and WQ, as expected from the working principle, no dependency was observed with fluctuation of the mark depth. The APD of the RAM shown in fig. 9 is substantially zero (<0.05nm), independent of the mark depth. The RAM used in fig. 9 and 10 is designed to work best at 850nm (far infrared, FIR), but the marks can be designed for other wavelengths, such as 635nm (red).

Fig. 11 depicts Alignment Position Deviation (APD) as a function of alignment mark depth in the presence of sidewall angle asymmetry for a conventional phase grating and two resonant amplitude gratings, respectively. The inset in fig. 11 illustrates the shape of the sidewall angle asymmetry. In this case, the sidewall angle is 1.1 °. The pitch of a conventional phase grating is 3.2 μm and the figure has square marks. The pitches of the resonant amplitude grating are 1.94 μm and 3.11 μm, respectively, and the figures have circular marks and triangular marks, respectively. The wavelength was 850 nm.

Fig. 12 shows Wafer Quality (WQ) as a function of alignment mark depth in the presence of sidewall angle asymmetry for a conventional phase grating and two resonant amplitude gratings, respectively.

As can be seen from fig. 11 and 12, SWA also has less effect on APD for RAM than for conventional tags. While the impact of SWA is generally limited for smaller mark depths, APD fluctuations as a function of mark depth are much smaller for RAM. This becomes critical in dealing with the fluctuations induced in the signature topography. Therefore, in this case, the RAM performance is also superior to the conventional flag.

Figure 13 depicts Alignment Position Deviation (APD) as a function of alignment mark depth in the presence of top tilt asymmetry for a conventional phase grating and two resonant amplitude gratings, respectively. The inset in fig. 13 illustrates the shape of the top tilt asymmetry. In this case, the top tilt is a height difference of 1nm from one side of the trench to the other. The pitch of a conventional phase grating is 3.2 μm and the figure has square marks. The pitches of the resonant amplitude grating are 1.94 μm and 3.11 μm, respectively, and the figures have circular marks and triangular marks, respectively. The wavelength is 850 nm.

Fig. 14 depicts Wafer Quality (WQ) as a function of alignment mark depth in the presence of top tilt asymmetry for a conventional phase grating and two resonant amplitude gratings, respectively.

As can be seen in fig. 13, for RAM, the effect of the top tilt is a constant non-zero APD. Although the performance is less clear than in the FT and SWA scenarios, it is meaningless that APDs do not fluctuate according to mark depth and therefore can be easily corrected unlike conventional marks.

Figure 15 depicts a resonant amplitude grating having a sub-divided portion of unit cells according to one embodiment of the present invention. Features that are the same as those described with reference to figure 5 have the same reference numerals. The first portion 1510 is subdivided to produce a first effective index n_s. In this example, the first portion is sub-divided by a periodic sub-structure 1511, the duty cycle of the periodic sub-structure 1511 being selected to generate a first effective refractive index n_s。

In another embodiment (not shown), the second portion may be sub-divided to generate a second effective refractive index n_d. The second portion may be subdivided by periodic substructures, the duty cycles of which are selected to generate the second effective refractive index.

FIG. 16 depicts a label stack with resonant amplitude labels on top according to one embodiment of the invention. Features that are the same as those described with reference to figure 5 have the same reference numerals. The

resonant amplitude gratings

510, 504 are formed on an intermediate layer 1612 on a stack of

gratings

1614, 1616, 1618 on a planar substrate 512. For a mark stack as depicted with reference to fig. 16, the RAM is less sensitive to the presence of other alignment marks in the underlying layers, since this mode propagates in the grating plane. This may reduce cross talk, allowing for more robust readout. This saves the space of the scribe lane.

FIG. 17 depicts an overlay target having bottom resonance amplitude markers according to an embodiment of the present invention. Features that are the same as those described with reference to figure 2 have the same reference numerals.

The overlay target has a resonant amplitude mark as a lower mark, with

periodic structures

1710, 1704 formed on a substrate 1712, such as described with reference to fig. 5. The mark has a periodic structure configured to scatter radiation 1702 of wavelength λ incident 200 on the surface plane of the alignment mark.

The lower marks overlap the upper marks, which have the same pitch as the lower marks and comprise a phase grating periodic structure 210 on the intermediate layer 1704. The periodic structure 210 is configured to scatter the radiation 202 without exciting resonant modes in the periodic structure that are parallel to its surface plane on which the radiation is incident. The scattering from this upper grating is mainly achieved by interference between the radiation reflected from the top and bottom of the grating. The reference numerals of the upper phase grating indicate the same features as described with reference to fig. 2.

As depicted in fig. 1, the overlay target is formed on a planar substrate such as a wafer W. The overlay target is insensitive to the asymmetry of the bottom grating because it uses the resonant amplitude marks as the bottom grating.

Fig. 18 is a flow diagram of an alignment method according to one embodiment of the invention. The alignment method has the following steps.

1802 (MRK): an alignment mark formed on a planar substrate is provided.

1804 (ILL): the alignment mark is irradiated with radiation of a predetermined wavelength.

1806 (DET): radiation scattered by the alignment marks resulting from the illumination is detected.

1808 (APD): the position of an alignment mark (APD) is determined using the detected radiation.

The alignment mark has a periodic structure configured to scatter radiation incident on a surface plane of the alignment mark. The surface plane is parallel to the plane of the substrate. Scattering is achieved primarily by exciting resonant modes in the periodic structure that are adjacent and parallel to the surface plane.

FIG. 19 is a flow diagram of a method of overlay error measurement according to one embodiment of the invention. The method of determining an overlay error has the following steps.

1802 (TGT): an overlay target formed on a planar substrate is provided. The overlay target has a lower mark overlapping an upper mark having the same pitch as that of the lower mark. The lower mark has a periodic structure configured to scatter radiation incident on a surface plane of the lower mark. The surface plane is parallel to the plane of the substrate. Scattering is achieved primarily by exciting resonant modes in their periodic structure that are adjacent parallel to their surface plane. The upper marker has a periodic structure configured to scatter radiation of a predetermined wavelength without exciting in its periodic structure a resonant mode that is parallel adjacent to its surface plane on which the radiation is incident.

1904 (ILL): the overlapping targets are illuminated with radiation of a predetermined wavelength.

1906 (DET): radiation scattered by overlapping targets caused by the illumination is detected.

1908 (OV): the detected radiation is used to determine the overlay error OV between the upper and lower marks.

Referring to fig. 18 and 19, the periodic structure has a repeating unit cell divided into adjacent first and second portions along the periodic direction. The first portion has a first effective index of refraction and a first length along the periodic direction. The second portion has a second effective refractive index lower than the first effective refractive index in its optical path, and a second length along the periodic direction. The lengths of these portions and the effective refractive index are configured to provide an optical path length of the unit cell in the periodic direction that is equal to an integer multiple of the wavelengths present in the radiation spectrum. The effective refractive indices and lengths of the portions are configured to provide an optical path length of the second portion in the periodic direction that is equal to half of an integer multiple of the wavelengths present in the radiation spectrum.

Preferably, the optical path length of the second portion in the direction of periodicity is equal to half the wavelength present in the radiation spectrum.

The first portion may be subdivided to produce a first effective index of refraction. For example, the first portion may be sub-divided by a periodic sub-structure having a duty cycle selected to generate the first effective refractive index.

The second portion may be sub-divided to generate a second effective index of refraction. For example, the second portion may be subdivided by periodic substructures, the duty cycles of which are selected to generate the second effective refractive index.

As described below with reference to fig. 20-23, the periodic structure may have a third portion interleaved with the repeating sequence of unit cells along the periodic direction, wherein the third portion has a third effective refractive index and a third length along the periodic direction that is greater than the first length. In this case, the effective refractive indices and lengths of the portions are configured to provide an optical path length in the periodic direction that is the sum of: a plurality of first portions; a plurality of second portions; and a third section, the optical path length being equal to an integer multiple of the wavelengths present in the radiation spectrum.

The third effective refractive index is generally equal to the first effective refractive index. The third portion may be sub-divided to generate a third effective index of refraction. The third portion may be subdivided by periodic substructures, the duty cycles of which are selected to generate a third effective refractive index.

As described with respect to fig. 5, for the RAM (resonance amplitude mark), the following condition is satisfied.

The optical length of the unit cell is equal to an integer multiple of the wavelength for efficient coupling to the resonant mode; and the optical path length of the low refractive index material (trench) is equal to half the wavelength for effective leakage of the mode in the sensor plane, which is defined by two equations:

n_sL₁+n_dL₂＝mλ

n_dL₂＝λ/2

wherein L is₁(L₂) Is the width of the ridge (groove), n_s(n_d) Is the diffraction index of the ridge (trench) material, and m is an integer (≧ 2). The main pitch of the marks being Λ ═ L₁+L₂.

As a result, for a fixed wavelength λ, the mark pitch Λ is related to the mark Duty Cycle (DC), which is defined as L₁And/Λ. However, in actual alignment of the marksIn applications, the DC of the RAM is higher than 75%.

The DC of the RAM can be increased by reducing the mark pitch a. However, the reduced pitch may not be compatible with the optical characteristics (e.g., numerical aperture) of the alignment sensor(s) utilized.

Large DC limits the use of RAM during mark design because mark parameters such as DC, pitch, and Critical Dimension (CD) are constrained due to semiconductor manufacturing process tolerances and variations. Furthermore, semiconductor manufacturers widely use sub-partitioned marks to reduce mark-to-device offset. Ridge (L) to RAM₁) Or groove (L)₂) Sub-dividing will produce even larger DC values.

In this embodiment, referring to fig. 20 to 23, a super-wavelength (supra-wavelength) sub-division mark of Resonance Amplitude (RASSM) is designed to solve the above-described duty ratio-related problem to extend the use of the Resonance Amplitude Mark (RAM) as described with reference to fig. 5 and 15 while maintaining advantages. By super-wavelength is meant along a unit cell (along L)₁+L₂) Has an effective length equal to or greater than the wavelength. The super-wavelength is used herein to distinguish from sub-wavelength sub-divisions, such as described with reference to fig. 15, where the sub-wavelength (and sub-resolution) sub-division has an optical path through the unit cell of the sub-division that is smaller than the wavelength.

Fig. 20 shows an example. The design rules in this example include:

1) the effective width of the super-wavelength sub-division trench is equal to half the wavelength:

n_dL₂＝λ/2；

2) the effective width of the super-wavelength subdivision pitch is equal to an integer multiple of the wavelength:

n_sL₁+n_dL₂＝m₁λ, wherein m₁Not less than 2; and

3) the effective width of the main pitch is equal to an integer multiple of the wavelength:

(N_g-1)n_sL₁+N_gn_dL₂+n_sL₃＝m₂λ, wherein N_g-1≥1，N_gIs not less than 2, and n_sL₃≥4.5λ

Wherein N is_gIs the number of grooves, m, in the unit cell grating that is repeated₂Is an integer. The main pitch of the marks is Λ m ═ (N)_g-1)L₁+N_gL₂+L₃. Main duty cycle of DC _ m ═ L₃and/Lambda _ m. The super-wavelength sub-division pitch is lambada _ swsubseg ═ L₁+L₂And the super-wavelength sub-division duty ratio is DC _ swsubseg ═ L₁/Λ_swsubseg。

Rules 2) and 3) result in efficient coupling to the resonant mode, whereas rule 1) results in efficient leakage of this mode in the grating plane, similar to the embodiment described with reference to fig. 5 and 15.

Fig. 20 depicts a resonant amplitude marker according to an embodiment of the present invention.

The marks are formed on a planar substrate 2012. Like the mark of fig. 5, the mark has a periodic structure configured to scatter radiation of wavelength λ incident on the surface plane of the alignment mark. The surface plane is parallel to the plane of the substrate. Scattering is mainly achieved by exciting resonant modes 2008 in the periodic structure parallel to the surface plane.

The periodic structure has a repeating unit cell divided into adjacent first and second portions 2010 and 2004 along a periodic direction (from left to right in the cross-section of fig. 20).

The first portion 2010 has a first effective index (n)_s) And a first length (L) along the periodic direction₁). The second portion 2004 has a second effective index (n) lower than the first effective index on its path along the periodic direction_d) And a second length (L) along the periodic direction₂)。

The periodic structure also has a third portion 2014 that is interleaved with the sequence of repeating unit cells along the periodic direction. The third portion has a third effective refractive index and a length (L) greater than the first length₁) Of a third length (L) along the periodic direction₃). In this example, the third effective index is equal toFirst effective refractive index (n)_s)。

Effective refractive indices (n) of the first and second portions_s，n_d) And length (L)₁，L₂) Is arranged in a periodic direction (n)_sL₁+n_dL₂) Providing an optical path length of the unit cell equal to an integer multiple (m) of a wavelength present in the radiation spectrum₁λ). In this example, m ₁2. Effective refractive index (n) of these portions_s，n_d) And length (L)₁，L₂，L₃) Further configured to provide an optical path length in the periodic direction that is the sum of: a plurality of first portions; a plurality of second portions; and a third portion, the optical path length being equal to an integer multiple (m) of the wavelengths present in the radiation spectrum₂λ). In this example, m ₂10, as follows.

Thus, in the example shown in fig. 20:

Λ_m＝(N_g-1)L₁+N_gL₂+L₃＝2L₁+3L₂+L₃

and replacing N with 3 (three grooves including a plurality of repeated unit cells)_gAnd using n_sL₁＝3λ/2，n_dL₂λ/2 and n_sL₃11 λ/2, the third design rule,

(N_g-1)n_sL₁+N_gn_dL₂+n_sL₃＝m₂λ, is

2n_sL₁+3n_dL₂+n_sL₃＝2x3λ/2+3xλ/2+11λ/2＝10λ.

Thus, m₂＝10

Effective refraction of the first and second portionsRate (n)_s，n_d) And length (L)₁，L₂) Is also configured to provide the second portion in a periodic direction (n)_dL₂) Equal to half an integer multiple of the wavelengths present in the radiation spectrum (k λ/2). These are conditions that match the wavelength of the radiation to the grating material boundary conditions to support resonance.

In this example, the optical path length of the second portion in the periodic direction (n)_dL₂) Equal to half the wavelength (lambda/2) present in the radiation spectrum, so that only one antinode of the resonant mode is present in the second part 504, i.e. k is 1. When k is>1, there are an odd number of antinodes, but an even number cancel out, leaving only one antinode that contributes to scattering but is less efficient.

The e portion may be sub-wavelength sub-divided to generate a third effective index in the same manner as described with reference to fig. 15. The third portion may be sub-wavelength sub-divided by a periodic sub-structure having a duty cycle selected to generate a third effective index of refraction.

Fig. 21 depicts a sub-wavelength sub-division phase mark compared to the resonance amplitude mark of fig. 20. The marks are formed on a planar substrate 2112. The length L of the ridges 2110 and grooves 2104 in FIG. 21₁And L₂Respectively, less than the length of fig. 20(2010 and 2004). The sub-division has an optical path through the unit cell of the sub-division, which is smaller than the wavelength, and therefore, there is not enough space in the groove to allow resonance. Like RASSM described with reference to FIG. 20, the periodic structure of FIG. 21 also has a length L₃And a third portion 2114 interleaved with the sequence of repeating unit cells along the periodic direction.

Fig. 22 depicts simulated Alignment Position Deviations (APDs) as a function of alignment mark depth for the resonant amplitude hyper-wavelength sub-division mark (RASSM) of fig. 20 and the sub-wavelength sub-division phase marks of fig. 21. APDs as a function of mark depth in the presence of a 2nm backplane tilt are shown for RASSM 2202(Λ _ swsubseg ═ 0.773 μm) and marks with a standard subdivision pitch 2204(Λ _ subseg ═ 0.246 μm), respectively. The main pitch (Λ _ m ═ 3.25 μm), main DC (DC _ m ═ 9.31), and subdivision DC (DC _ subset ═ 45.04%) of the two marks are the same. The wavelength is 850nm with TE polarization.

Fig. 23 depicts simulated Wafer Quality (WQ) as a function of alignment mark depth for the resonant amplitude hyper-wavelength sub-division mark (RASSM) of fig. 20 and the sub-wavelength sub-division phase mark of fig. 21. WQ as a function of mark depth in the presence of a 2nm floor tilt is shown for RASSM 2302(Λ _ swsubseg ═ 0.773 μm) and for marks with a standard sub-division pitch 2304(Λ _ subseg ═ 0.246 μm), respectively. The main pitch (Λ _ m ═ 3.25 μm), main DC (DC _ m ═ 39.31%), and sub-division DC (DC _ subdivision ═ DC _ swsubdivision ═ 45.04%) of the two marks were the same. The wavelength is 850nm with TE polarization.

As can be seen from the simulation results shown in fig. 22 and 23, the APDs and WQs for the RASSM super-wavelength sub-division marks 2202, 2302, respectively, are compared with those of the standard sub-wavelength sub-division marks 2204, 2304, where the main pitch (Λ _ m), the main DC (DC _ m), and the sub-division DC (DC _ subseg ═ DC _ swsubseg) are the same. Compared to standard markers, the APD of RASSM is more stable with less variation, while the Wafer Quality (WQ) of RASSM is much higher.

The RASSM of fig. 20 has been compared to another signature with a smaller superwavelength sub-division DC but with the same main pitch, main DC and superwavelength sub-division pitch. This means that for another label, L₂Is increased by L₁By the same amount. In this case, both APD and WQ of RASSM are more stable and vary less compared to other labels with smaller ultrawavelength sub-divided DC. This is because the design rule is not implemented as n_dL₂>λ/2。

During this study, it was also found that L was present only₂No more than half the wavelength, the variation of APD and WQ is small, but WQ varies with L₂Is reduced. Thus, design rule 1 is still valid for optimal RASSM design. For labels with larger sub-divided DC, APD performance is as good as optimized RASSM, while its WQ is stable but lower. The reason is that when L is₂Equal to or less than half of the wavelength, the trench widthThe degree is too small for the incident electric field to enter. Also, leakage from modes (or modi) of adjacent ridges will not be coupled. As a result, the electric field is only present outside the trench and is insensitive to both mark depth and mark asymmetry. When the sub-division DC is large, the effective index contrast of the grating material is smaller, thereby reducing the WQ.

The RASSM of figure 20 can be used in stacked gratings and overlapping targets respectively in the same manner as described with reference to figures 16 and 17 respectively.

Embodiments of the present invention have several advantages. The alignment and overlay measurement methods are less complex; for alignment purposes, only a single wavelength is required, since for RAM, the WQ and APD do not vary as a function of mark depth.

The alignment measurement method and the overlay measurement method are more accurate; in the case of process-induced asymmetries, especially for FT, the APD obtained for RAM is extremely small; for a typical value of FT ═ 1nm, APD is less than 0.5 angstroms.

The alignment measurement method and the overlay measurement method are faster, especially in case an adjustable light source of only one color can be provided at a time.

The marks and targets can be used for golden reference wafers for wafer-to-wafer error correction because they are not sensitive to asymmetry.

The copper dual damascene structure is improved; the presence of layers below the RAM marks has a limited effect on the signal, thus allowing a more reliable APD or OV readout.

As described with reference to fig. 17, the RAM may be used as a bottom grating for overlay targets to reduce the effect of layers under the marks that affect the overlay readout signal.

Still further, embodiments of the present invention are compatible with smaller tags.

The RASSM embodiments described with reference to fig. 20-23 increase the duty cycle, pitch and selectivity of sub-division of the RAM mark design. It also provides more stable APDs and WQs compared to conventional sub-division labels. It is less sensitive to varying mark depths than conventional sub-segmented marks. This gives more freedom and selectivity in the alignment sensor design. The RASSM embodiment extends the use of RAM to accommodate practical semiconductor manufacturing process tolerances and variations.

Other embodiments are disclosed below in the numbered list of embodiments.

1. A marker formed on a planar substrate, the marker comprising a periodic structure configured to scatter radiation incident on a surface plane of the alignment marker, the surface plane being parallel to the plane of the substrate, the scattering being achieved primarily by excitation of resonant modes in the periodic structure that are parallel to the surface plane.

2. The mark according to embodiment 1, wherein the periodic structure has a repeating unit cell divided into adjacent first and second portions along the periodic direction,

-the first portion has a first effective refractive index and a first length along the periodic direction;

the second portion has a second effective refractive index lower than the first effective refractive index on its optical path and a second length along the periodic direction;

wherein the effective refractive index and the length of the portions are configured to provide:

-an optical path length of the unit cell in the periodic direction, equal to an integer multiple of the wavelengths present in the spectrum of the radiation; and

-the optical path length of the second portion in the direction of periodicity being equal to half of an integer multiple of the wavelengths present in the spectrum of the radiation.

3. A tag as in embodiment 2, wherein the optical path length of the second portion in the periodic direction is equal to half the wavelength present in the spectrum of the radiation.

4. The mark of

embodiment

2 or 3 wherein the first portion is subdivided to produce a first effective index.

5. The marker of embodiment 4, wherein the first portion is subdivided by a periodic substructure, a duty cycle of the periodic substructure being selected to generate the first effective index of refraction.

6. The marking of any one of embodiments 2 to 5, wherein the second portion is sub-divided to generate the second effective refractive index.

7. The marker of embodiment 6, wherein the second portion is sub-divided by a periodic sub-structure having a duty cycle selected to generate the second effective index of refraction.

8. The mark according to any one of embodiments 2 to 7, wherein the periodic structure has a third portion that is interleaved with the repeating sequence of unit cells along the periodic direction, wherein

-the third portion has a third effective refractive index and a third length along the periodic direction that is greater than the first length; and

-an optical path length in the periodic direction of the sum of: a plurality of first sections, a plurality of second sections, and a third section, the optical path length being equal to an integer multiple of wavelengths present in the spectrum of radiation.

9. The marking of embodiment 8, wherein the third effective refractive index is equal to the first effective refractive index.

10. The mark of embodiment 8 or embodiment 9, wherein the third portion is subdivided to produce a third effective index.

11. The marker of embodiment 10, wherein the third portion is subdivided by periodic substructures, the duty cycles of the periodic substructures being selected to generate the third effective refractive index.

12. A substrate comprising a marking according to any preceding embodiment.

13. An overlay target comprising a lower mark according to any of embodiments 1-12, the lower mark overlapping an upper mark, the upper mark having the same pitch as the lower mark and comprising a periodic structure configured to scatter radiation without exciting in the periodic structure a resonant mode parallel to its surface plane on which the radiation is incident.

14. A substrate comprising the overlay target of embodiment 13.

15. An alignment method comprising the steps of:

-illuminating the alignment mark with radiation;

-detecting radiation scattered by the alignment mark resulting from the illumination;

-determining the position of the alignment mark using the detected radiation.

16. The method of embodiment 15, wherein the periodic structure has a repeating unit cell divided into adjacent first and second portions along the periodic direction,

-the first portion has a first length along the periodic direction and a first effective refractive index;

17. The method of embodiment 15 or embodiment 16, wherein an optical path length of the second portion in the periodic direction is equal to half a wavelength present in a spectrum of the radiation.

18. The method of any of embodiments 15-17, wherein the first portion is subdivided to generate the first effective index.

19. The method of embodiment 18 wherein the first portion is subdivided by a periodic substructure, the duty cycle of the periodic substructure being selected to generate the first effective index.

20. The method of any of embodiments 15-19, wherein the second portion is subdivided to generate the second effective index.

21. The method of embodiment 20 wherein the second portion is subdivided by a periodic substructure, the duty cycle of the periodic substructure being selected to generate the second effective index of refraction.

22. The method of any of embodiments 15-21, wherein the periodic structure has a third portion that is interleaved with the sequence of repeating unit cells along the periodic direction, wherein

-an optical path length in the periodic direction of the sum of: a plurality of first portions; a plurality of second portions; and a third section, the optical path length being equal to an integer multiple of wavelengths present in the spectrum of radiation.

23. The method of embodiment 22, wherein the third effective index is equal to the first effective index.

24. The method of embodiment 22 or embodiment 23, wherein the third portion is subdivided to generate a third effective index.

25. The method of embodiment 24 wherein the third portion is subdivided by periodic substructures, the duty cycles of the periodic substructures selected to generate the third effective index of refraction.

26. A method of determining overlay error, comprising the steps of:

The lower mark comprises a periodic structure configured to scatter radiation incident on a surface plane of the lower mark, the surface plane being parallel to the plane of the substrate, the scattering being mainly achieved by exciting resonant modes in its periodic structure that are parallel to its surface plane; and

the upper marker comprises a periodic structure configured to scatter the radiation without exciting in its periodic structure a resonance mode parallel to its surface plane on which the radiation is incident;

-illuminating the overlapping targets with radiation;

27. The method of embodiment 26, wherein the periodic structure has a repeating unit cell divided into adjacent first and second portions along the periodic direction,

28. The method of embodiment 26 or embodiment 27, wherein an optical path length of the second portion in the periodic direction is equal to half a wavelength present in a spectrum of the radiation.

29. The method of any of embodiments 26-28, wherein the first portion is subdivided to generate the first effective index.

30. The method of embodiment 29 wherein the first portion is subdivided by a periodic substructure, the duty cycle of the periodic substructure being selected to generate the first effective index.

31. The method of any of embodiments 26-30, wherein the second portion is subdivided to generate the second effective index.

32. The method of embodiment 31 wherein the second portion is subdivided by a periodic substructure, the duty cycle of the periodic substructure being selected to generate the second effective index of refraction.

33. The method of any of embodiments 26 through 32, wherein the periodic structure has a third portion that is interleaved with the sequence of repeating unit cells along the periodic direction, wherein

34. The method of embodiment 33, wherein the third effective index is equal to the first effective index.

35. The method of embodiment 33 or embodiment 34, wherein the third portion is subdivided to generate a third effective index.

36. The method of embodiment 35, wherein the third portion is subdivided by a periodic substructure, the duty cycle of the periodic substructure being selected to generate the third effective index of refraction.

37. A marker on a substrate comprising a structure configured to scatter radiation incident on a surface plane of the marker, the scattering being achieved primarily by excitation of resonant modes in the structure parallel to the surface plane.

38. A substrate comprising a marking according to any preceding embodiment.

39. An alignment method comprising the steps of:

-irradiating the substrate according to embodiment 38 with radiation;

-detecting radiation scattered by the label caused by the illumination; and

-determining the position of the marker using the detected radiation.

40. A method of determining overlay error, comprising the steps of:

-illuminating the overlapping targets according to embodiment 13 with radiation;

-detecting radiation scattered by overlapping targets caused by the illumination;

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the treatment of substrates in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Those skilled in the art will appreciate that any use of the terms "wafer" or "field"/"die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively, in the context of such alternative applications. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example, in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use of embodiments of the disclosure in the context of optical lithography, it will be appreciated that the disclosure may be used in other applications, for example imprint lithography, and where the context allows, is not limited to lithography. In the case of imprint lithography, the topography in the patterning device defines the pattern created on the substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate, and the resist is then cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in the resist after it has cured.

The terms "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including Ultraviolet (UV) radiation (e.g. having a wavelength of or about 365nm, 248nm, 193nm, 157nm or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of between 5nm and 20 nm), as well as particle beams, such as ion beams or electron beams.

The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above; or in the form of a data storage medium (e.g., semiconductor memory, magnetic or optical disk) having such a computer program stored therein.

The above description is intended to be illustrative, and not restrictive. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below. In addition, it should be appreciated that structural features or method steps shown or described in any embodiment herein may also be used in other embodiments.

Claims

1. A mark formed on a planar substrate, the mark comprising a periodic structure configured to scatter radiation incident on a surface plane of the mark, the surface plane being parallel to the plane of the substrate, wherein the periodic structure has a repeating unit cell divided along a periodic direction into adjacent first and second portions,

-the second portion has a second effective refractive index lower than the first effective refractive index on its optical path, and a second length along the periodic direction;

wherein the effective refractive indices and lengths of the first and second portions are configured to provide:

-an optical path length of the repeating unit cell in the periodic direction, which is equal to an integer multiple of wavelengths present in the spectrum of the radiation; and

-an optical path length of the second portion in the periodic direction, which is equal to half of an integer multiple of the wavelengths present in the spectrum of the radiation.

2. A tag according to claim 1, wherein the optical path length of the second portion in the periodic direction is equal to half the wavelength present in the spectrum of the radiation.

3. The marking as claimed in claim 1, wherein the first portion is sub-divided to generate the first effective refractive index.

4. The marking of claim 1, wherein the first portion is subdivided by periodic substructures, a duty cycle of the periodic substructures being selected to generate the first effective refractive index.

5. The marking as claimed in claim 1, wherein the second portion is sub-divided to generate the second effective refractive index.

6. The marking as claimed in claim 5, wherein the second portion is subdivided by periodic substructures, the duty cycles of which are selected to generate the second effective refractive index.

7. The tag of claim 1, wherein the periodic structure has a third portion that is interleaved with the sequence of repeating unit cells along the periodic direction, wherein

wherein the effective refractive indices and lengths of the first, second and third portions are configured to provide:

-an optical path length in the periodic direction of the sum of: a plurality of said first portions; a plurality of said second portions; and the third section, the optical path length being equal to an integer multiple of wavelengths present in the spectrum of the radiation.

8. An overlay target comprising a lower mark, the lower mark being a mark according to claim 1, the lower mark overlapping an upper mark, the upper mark having the same pitch as the lower mark.

9. A method of designing a mark comprising a periodic structure configured to scatter radiation incident on a surface plane of the mark, the surface plane being parallel to a plane of a substrate, wherein the periodic structure has a repeating unit cell divided into adjacent first and second portions along a periodic direction, the method comprising:

-determining a first length and a first effective refractive index along the periodic direction of the first portion;

-determining a second length along the periodic direction of the second portion and a second effective refractive index;

wherein the design of the mark comprises the determined effective refractive indices and lengths of the first and second portions, wherein the determination is based on:

-adjusting the optical path length of the repeating unit cell along the periodic direction to an integer multiple of the wavelengths present in the spectrum of the radiation; and

-adjusting the optical path length of the second portion along the periodic direction to half of an integer multiple of the wavelengths present in the spectrum of the radiation.

10. A method of selecting wavelengths present in a spectrum of radiation incident on a surface plane of a mark on a substrate, the surface plane being parallel to the plane of the substrate, the mark comprising a periodic structure having a repeating unit cell divided into adjacent first and second portions along a periodic direction, the method comprising:

-obtaining an optical path length of the repeating unit cell along the periodic direction;

-obtaining an optical path length of the second portion along the periodic direction; and

-selecting wavelengths comprised in said spectrum for which said optical path length of said repeating unit cell along said periodic direction equals an integer multiple of said wavelength and said optical path length of said second portion along said periodic direction equals half of an integer multiple of said wavelength.

11. An alignment method comprising the steps of:

-providing an alignment mark formed on a planar substrate, the alignment mark being designed according to the method of claim 9;

-illuminating the alignment mark with radiation;

-detecting radiation scattered by the alignment mark; and

-determining the position of the alignment mark using the detected radiation.

12. A method of determining overlay error, comprising the steps of:

-providing an overlay target formed on a planar substrate, the overlay target being a target according to claim 8;

-illuminating the overlay target with radiation;

-detecting radiation scattered by the overlapping targets; and

-determining an overlay error using the detected radiation.

13. A substrate comprising the marking according to claim 1.

14. A computer program containing one or more sequences of machine-readable instructions which specify the method of claim 9.

15. A data storage medium having stored therein a computer program according to claim 14.